Programming in C

Chars, Strings and StructsChars, Strings and Structs

ASCII• The American Standard Code for Information Interchange The American Standard Code for Information Interchange (ASCII)(ASCII)

character set, has 128 characters designed to encode the Roman character set, has 128 characters designed to encode the Roman alphabet used in English and other Western European languages. alphabet used in English and other Western European languages.

• C was designed to work with ASCII and we will only use ASCII in this C was designed to work with ASCII and we will only use ASCII in this course. ASCII can represent 128 characters and is encoded in one course. ASCII can represent 128 characters and is encoded in one eight bit byte with a leading 0. Seven bits can encode numbers 0 to eight bit byte with a leading 0. Seven bits can encode numbers 0 to 127. Since integers in the range of 0 to 127 can be stored in 1 byte of 127. Since integers in the range of 0 to 127 can be stored in 1 byte of space, the space, the sizeof(char)sizeof(char) is 1. is 1.

• The characters 0 through 31 represent The characters 0 through 31 represent control characterscontrol characters (e.g., line (e.g., line feed, back space), 32-126 are feed, back space), 32-126 are printable charactersprintable characters, and 127 is , and 127 is deletedelete . .

char type

• Like Java, C supports the Like Java, C supports the char data type for storing data type for storing a single character.a single character.

• char uses one byte of memory uses one byte of memory• char constants are enclosed in single quotes, ‘A’ constants are enclosed in single quotes, ‘A’• Use Use %c in in printf( ) to print a single character to print a single character

ASCII Character Chart

ASCII Observations• Characters for digits are consecutiveCharacters for digits are consecutive

– Can change an ASCII digit to its decimal value by subtracting ‘0’• ASCII digit - ‘0’ = digit value (e.g. ‘7’ - ‘0’ = 7)

– Can use a loopfor( d = ‘0’; d <= ‘9’; d++) printf( “%c”, d);

• Uppercase and lowercase letters are consecutiveUppercase and lowercase letters are consecutive– Can use a loop

for (k = 0; k < 26; k++) {printf( “%c”, ‘A’ + k);printf( “%c”, ‘a’ + k);

}for (c = ‘A’; c <= ‘Z’; c++)

printf( “%c”, c);

– Can use them as an indexchar grade = ‘D’;

int x = score [ grade - ‘A’];

Special Characters• The backslash character, \, is used to indicate that the char that The backslash character, \, is used to indicate that the char that

follows has special meaning. E.g. for unprintable characters follows has special meaning. E.g. for unprintable characters and special characters.and special characters.

• For exampleFor example– \n is the newline character– \t is the tab character– \” is the double quote (necessary since double quotes are used to

enclose strings)– \’ is the single quote (necessary since single quotes are used to

enclose chars)– \\ is the backslash (necessary since \ now has special meaning)– \a is beep which is unprintable

Special Char Example Code• What is the output from these statements?What is the output from these statements?

• printf(“\t\tMove over\n\nWorld, here I come\n") ;printf(“\t\tMove over\n\nWorld, here I come\n") ;

Move over

World, here I come

• printf("I\’ve written \”Hello World\”\n\t many times\n\a“); printf("I\’ve written \”Hello World\”\n\t many times\n\a“);

I’ve written “Hello World”many times <beep>

Character Library

• There are many functions written to handle characters. To use There are many functions written to handle characters. To use these functions, include these functions, include <ctype.h>.

• Note that the function parameter type is Note that the function parameter type is int, not , not char. Why is . Why is this ok?this ok?

• Note that the return type for some functions is Note that the return type for some functions is int since C since C does not support does not support boolean..

• A few of the commonly used functions are listed on the next A few of the commonly used functions are listed on the next slide. For a full list of slide. For a full list of ctype.h functions, type functions, type man ctype.h at at the unix prompt.the unix prompt.

ctype.h• int isdigit (int c); int isdigit (int c);

– Determine if c is a decimal digit (‘0’ thru ‘9’)

• Int isxdigit(int c);Int isxdigit(int c);– Determines if c is a hexadecimal digit (‘0’ - ’9’, ‘a’ - f’, ‘A’ - ‘F’)

• int isalpha (int c);int isalpha (int c);– Determines if c is an alphabetic character (upper or lower-case)

• int isspace (int c);int isspace (int c);– Determines if c is a whitespace character (space, tab, etc)

• int isprint (int c); int isprint (int c); – Determines if c is a printable character

• int tolower (int c);int tolower (int c);• int toupper (int c); int toupper (int c);

– Changes c to lower- or upper-case respectively, if possible

Strings in C• In C, a string is an array of characters terminated with the In C, a string is an array of characters terminated with the

“null” character (‘\0’, value = 0).“null” character (‘\0’, value = 0).• Char arrays are permitted a special initialization using a string Char arrays are permitted a special initialization using a string

constant constant

char name[4] = “bob”;

which is shorthand for, but equivalent towhich is shorthand for, but equivalent to

char name[4] = {‘b’, ‘o’, ‘b’, ‘\0’};

‘b’ ‘o’ ‘b’ ‘\0’name

Let the compiler count

• If the size of your initialized char array is not specified, the If the size of your initialized char array is not specified, the compiler will count the characters and size your array for youcompiler will count the characters and size your array for youchar name[ ] = “bob”;

char title[ ] = “Mr.”;

title

‘b’ ‘o’ ‘b’ ‘\0’name

‘M’ ‘r’ ‘.’ ‘\0’

C String Library

• C provides a library of string functions. Note that C provides a library of string functions. Note that assignment( = ) and equality (==) operators don’t do the assignment( = ) and equality (==) operators don’t do the job.job.

• To use the string functions, include To use the string functions, include <string.h>.• Some of the more common functions are listed here on the Some of the more common functions are listed here on the

next slides.next slides.• To see all the string functions, type To see all the string functions, type man string.h at the at the

unix prompt.unix prompt.

C String Library (2)

• Commonly used string functionsCommonly used string functions– strlen( char string[ ] )

• Returns the number of characters in the string, not including the “null” character.

– strcpy( char s1[ ], char s2[ ] )• Copies s2 on top of s1. The order of the parameters mimics

the assignment operator.

– strcmp ( char s1[ ] , char s2[ ] )• Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically

– strcat( char s1[ ] , char s2[ ])• Appends (concatenates) s2 to s1

C String Library (3)

• Some function in the C String library have an Some function in the C String library have an additional size parameter.additional size parameter.– strncpy( char s1[ ], char s2[ ], int n )

• Copies at most n characters of s2 on top of s1. Again, the order of the parameters mimics the assignment operator.

– strncmp ( char s1[ ] , char s2[ ], int n )• Compares up to n characters of s1 with s2.

• Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically

– strncat( char s1[ ], char s2[ ] , int n)• Appends at most n characters of s2 to s1

• Use Use %s%s in in printf( )printf( ) to print a string. to print a string.

String Codechar first[10] = “bobby”;char last[15] = “smith”;char name[30];char you[5] = “bobo”;

strcpy( name, first );strcat( name, last );printf( “%d, %s\n”, strlen(name), name );

strncpy( name, last, 2 );printf( “%d, %s\n”, strlen(name), name );

int result = strcmp( you, first );result = strncmp( you, first, 3 );strcat( first, last );

Simple Encryptionchar c, msg[] = "this is a secret message";int i = 0;char code[26] = /* Initialize our encryption code */ {'t','f','h','x','q','j','e','m','u','p','i','d','c','k','v','b','a','o','l','r','z','w','g','n','s','y'} ;

/* Print the original phrase */printf ("Original phrase: %s\n", msg);

/* Encrypt */while( msg[ i ] != '\0‘ ){

if( isalpha( msg[ i ] ) ) {c = tolower( msg[ i ] ) ;msg[ i ] = code[ c - ‘a’ ] ;

}++i;

}printf("Encrypted: %s\n", msg ) ;

“Big Enough”

• The “owner” of a string is responsible for allocating The “owner” of a string is responsible for allocating array space which is “big enough” to store the string array space which is “big enough” to store the string (including the null character)(including the null character)

• Most string library functions Most string library functions do notdo not check the size of check the size of the string memory. E.g. the string memory. E.g. strcpy

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What can happen?int main( ){ char first[10] = "bobby"; char last[15] = "smith";

printf("first contains %d chars: %s\n", strlen(first), first); printf("last contains %d chars: %s\n", strlen(last), last);

strcpy(first, "1234567890123"); /* too big */

printf("first contains %d chars: %s\n", strlen(first), first); printf("last contains %d chars: %s\n", strlen(last), last);

return 0;}

/* output */first contains 5 chars: bobbylast contains 5 chars: smithfirst contains 13 chars: 1234567890123last contains 5 chars: smithSegmentation fault

No Classes in C

• Because C is not an OOP language, there is no way to combine data and Because C is not an OOP language, there is no way to combine data and code into a single entity. C does allow us to combine related data into a code into a single entity. C does allow us to combine related data into a structure using the keyword structure using the keyword structstruct. All data in a . All data in a structstruct can be accessed can be accessed by any code. The general form of a structure definition is by any code. The general form of a structure definition is

struct tag{ member1_declaration; member2_declaration; member3_declaration; . . . memberN_declaration; };

where where struct is the keyword, is the keyword, tagtag names this kind of struct, and names this kind of struct, and member_declarations are variable declarations which define the are variable declarations which define the members. members.

Note the semi-colon

C struct Example• Defining a Defining a struct to represent a point in a coordinate plane to represent a point in a coordinate plane

struct point{

int x; /* x-coordinate */int y; /* y-coordinate */

};

• Given the declarations Given the declarations

struct point p1;struct point p2;

we can access the members of these we can access the members of these struct variables: variables:

* the x-coordinate of p1 is * the x-coordinate of p1 is * the y-coordinate of p1 is* the y-coordinate of p1 is* the x-coordinate of p2 is* the x-coordinate of p2 is* the y-coordinate of p2 is* the y-coordinate of p2 is

point is the struct tag

p1.xp1.yp2.xp2.y

Using structs and members

• Like other variable types, Like other variable types, struct variables (e.g. p1, p2) can variables (e.g. p1, p2) can be passed to functions as parameters and returned from be passed to functions as parameters and returned from functions as return types.functions as return types.

• The members of a The members of a struct are variables just like any other and are variables just like any other and ca be used wherever any other variable of the same type may ca be used wherever any other variable of the same type may be used. For example, the members of the be used. For example, the members of the struct point point can can then be used just like any other integer variables.then be used just like any other integer variables.

printPoint.cstruct point inputPoint( ){

struct point p;printf(“please input the x- and y-coordinates: “);scanf(“%d %d”, &p.x, &p.y);return p;

}

void printPoint( struct point point ){

printf (“( %2d, %2d )”, point.x, point.y);}

int main ( ){

struct point endpoint;endpoint = inputPoint( );printPoint( endpoint );return 0;

}

struct assignment

• The contents of a The contents of a struct variable may be copied to another variable may be copied to another struct variable of the same type using the assignment (=) variable of the same type using the assignment (=) operatoroperator

• After this code is executedAfter this code is executed

struct point p1;struct point p2;p1.x = 42;p1.y = 59;

p2 = p1; /* structure assignment copies members */

The values of The values of p2’s members are the same as ’s members are the same as p1’s members.’s members.E.g. E.g. p1.x = p2.x = 42 and and p1.y = p2.y = 59

struct within a struct• A data element in a A data element in a struct may be another may be another struct

(similar to composition in Java / C++).(similar to composition in Java / C++).• This example defines a line in the coordinate plane by This example defines a line in the coordinate plane by

specifying its endpoints as POINT specifying its endpoints as POINT structs

struct line{struct point leftEndPoint;struct point rightEndPoint;

};

• Given the declarations below, how do we access the x- and Given the declarations below, how do we access the x- and y-coodinates of each line’s endpoints?y-coodinates of each line’s endpoints?

struct line line1, line2;line1.leftEndPoint.x line1.rightEndPoint.x

line2.leftEndPoint.x line2.rightEndPoint.x

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Arrays of struct

• Since a Since a struct is a variable type, we can create arrays of is a variable type, we can create arrays of structs just like we create arrays of int, char, double, etc. just like we create arrays of int, char, double, etc.

• Write the declaration for an array of 5 line structures name Write the declaration for an array of 5 line structures name “lines”“lines”

struct line lines[ 5 ];

• Write the code to print the x-coordinate of the left end Write the code to print the x-coordinate of the left end point of the 3rd line in the arraypoint of the 3rd line in the array

printf( “%d\n”, lines[2].leftEndPoint.x);

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Array of struct Code/* assume same point and line struct definitions */

int main( )

{

struct line lines[5];

int k;

/* write code to initialize all data members to zero */

for (k = 0; k < 5; k++)

{

lines[k].leftEndPoint.x = 0;

lines[k].leftEndPoint.y = 0;

lines[k].rightEndPoint.x = 0;

lines[k].rightEndPoint.y = 0;

}

/* call the printPoint( ) function to print

** the left end point of the 3rd line */

printPoint( lines[2].lefEndPoint);

return 0;

}

Bitfields

• When saving space in memory or a communications When saving space in memory or a communications message is of paramount importance, we sometimes need message is of paramount importance, we sometimes need to pack lots of information into a small space. We can use to pack lots of information into a small space. We can use struct syntax to define “variables” which are as small as 1 syntax to define “variables” which are as small as 1 bit in size. These variables are known as “bit in size. These variables are known as “bit fields”.”.

struct weather

{unsigned int temperature : 5;

unsigned int windSpeed : 6;

unsigned int isRaining : 1;

unsigned int isSunny : 1;

unsigned int isSnowing : 1;

};

Using Bitfields

struct weather todaysWeather;

todaysWeather.temperature = 44;todaysWeather.isSnowing = 0;todaysWeather.windSpeed = 23;/* etc */

if (todaysWeather.isRaining)printf( “%s\n”, “Take your umbrella”);

if (todaysWeather.temperature < 32 )printf( “%s\n”, “Stay home”);

More on Bit fields

• Almost everything about bit fields is implementation Almost everything about bit fields is implementation (machine and compiler) specific.(machine and compiler) specific.

• Bit fields may only defined as Bit fields may only defined as (unsigned) ints• Bit fields do not have addresses, so the Bit fields do not have addresses, so the & operator operator

cannot be applied to themcannot be applied to them

• We’ll see more on this laterWe’ll see more on this later

Unions

• A A union is a variable type that may hold different is a variable type that may hold different type of members of different sizes, BUT only one type of members of different sizes, BUT only one type at a time. type at a time. All members of the union share the same memory. The compiler assigns enough . The compiler assigns enough memory for the largest of the member types.memory for the largest of the member types.

• The syntax for defining a The syntax for defining a union and using its and using its members is the same as the syntax for a members is the same as the syntax for a struct..

Formal Union Definition

• The general form of a The general form of a unionunion definition is definition is union tag{ member1_declaration; member2_declaration; member3_declaration; . . . memberN_declaration; };

where where union is the keyword, is the keyword, tagtag names this kind of names this kind of union, , and and mmember_declarations are variable declarations are variable declarations which define the members. Note that the syntax for defining a which define the members. Note that the syntax for defining a union is exactly the same as the syntax for a is exactly the same as the syntax for a struct..

union.c

union data{

int x;char c[8];

} ;

int i;union data item;

item.x = 42;printf(“%d, %o, %x”, item.x, item.x, item.x );for (i = 0; i < 8; i++ )

printf(“%x ”, item.c[i]);printf( “%s”, “\n”);printf(“%s\n”, “size of DATA = ”, sizeof(DATA));

Union vs. Struct

• SimilaritiesSimilarities– Definition syntax virtually identical– Member access syntax identical

• DifferencesDifferences– Members of a struct each have their own address in

memory. – The size of a struct is at least as big as the sum of the

sizes of the members (more on this later)– Members of a union share the same memory. The size of a

union is the size of the largest member.